In the latest violence, at least 13 people were killed in an air strike on a Damascus suburb, activists said.

The Syrian Observatory for Human Rights said at least eight children aged between six months and nine years old were among those killed in the attack on the rebel-held town of Muadhamiya. Several people were trapped under the rubble, the UK-based activist group added.

State television blamed "terrorists" for the deaths, saying they had fired a mortar at a civilian building.

The UN estimates that more than 60,000 people have been killed in the uprising, which began in March 2011.

One aid worker told us people were 'swimming in their tents' as snows melted

In addition to those who have left the country, at least two million people are thought be internally displaced within Syria.

'Drastically insufficient'

The International Rescue Committee says many refugees are citing rape as the main reason they left Syria and describes it as a major feature of the war. It is often committed in front of family members, it adds.

The agency criticises what is says is an "alarming" lack of medical and psychological support for survivors.

It describes international aid as "drastically insufficient" for a "steadily worsening" crisis.

The IRC speaks of whole neighbourhoods reduced to rubble and of displaced people moving from one village to another to escape a moving frontline.

In recent days the region has been hit by the worst winter storms in twenty years, making conditions for refugees even more desperate.

Among one group of Syrian refugees in northern Lebanon's Bekaa valley, not a single child was dressed for winter, the BBC's Lyse Doucet reports.

All were shivering, and coughing in the cold, with some even going without shoes, our correspondent says.

Last week, a UN agency warned that one million Syrians are going hungry.

The World Food Program (WFP) said it is helping 1.5 million Syrians, but continued fighting and an inability to use the port of Tartus to deliver food mean many people are not receiving aid.

'Coup d'etat'

The group said evidence indicated that Syrian forces had used ground-based BM-21 Grad multi-barrel rocket launchers to fire cluster bombs near the city of Idlib in December and north-west of Hama this month. Activists have previously reported the use of air-dropped cluster bombs.

The army was "now resorting to a notoriously indiscriminate type of cluster munition that gravely threatens civilian populations", said Steve Goose, director of HRW's arms division.

Nato earlier said Syrian government forces had launched a short-range, Scud-style ballistic missile on Sunday, bringing to more than 20 the number launched in the past month.

HRW published a photo of an unexploded bomblet delivered by a 122mm rocket used in one attack

And at least 55 governments have signed a letter demanding that the UN Security Council refer the conflict in Syria to the International Criminal Court at The Hague.

The initiative, which correspondents say has little chance of immediate success because of expected opposition from Russia and China, was organised by Switzerland.

In a separate development, Syria's deputy foreign minister told the BBC that President Bashar al-Assad intended to play a role in any potential transition government and would stand for re-election in 2014.

"The president, and many other candidates who may run, will go to the people, put [forward] their programmes, and be elected by the people. The ballot box will be where the future of the leadership of Syria will be decided," Faisal Mekdad added.

"It is a coup d'etat if you listen to what the armed groups and those enemies of Syria are proposing."

He said that a plan outlined by the president in a rare speech last week was credible and reflected Syrians' wish for national dialogue.

Mr Assad's plan was rejected by the opposition as empty rhetoric and described as a "missed opportunity" by the UN envoy, Lakhdar Brahimi.